A superconducting cable is attracting attention as a new electric power cable which is compact and is able to transmit a large amount of electric power with low loss. The superconducting cable generally includes a superconducting conductor layer, an insulating layer and a superconducting shield layer being layered around a flexible former in order. The superconducting conductor layer is configured by winding a plurality of tape shaped oxide superconducting wires. The insulating layer is configured by winding an insulating tape. The superconducting shield layer is configured by winding a plurality of tape shaped oxide superconducting wires.
Patent Document 1 discloses a superconducting cable wherein a superconducting conductor layer 3a, an insulating layer 51a, a superconducting shield layer 7a and the like are layered around a former 1a in order as shown in FIG. 4. A yttrium based oxide superconducting wire rod such as Y1Ba2Cu3O7-x (0≦X<1) is used as the tape shaped oxide superconducting wire rod for the superconducting conductor layer 3a, for example.
The insulating layer 51a is configured by winding insulating paper such as synthetic paper, semisynthetic insulating paper or kraft paper. The insulating layer 51a holds liquid nitrogen inside the insulating paper and in gaps (bad gap) between the sheets of the insulating paper when the superconducting cable is cooled by liquid nitrogen.
The numerical expression 1 represents electric field distribution in the insulating layer 51a which is formed of a uniform insulating material.
The withstand voltage performance of the insulating layer 51a needs to be higher than the maximum electric field in the insulating layer 51a. According to the following expression 1, the electric field in the insulating layer 51a reaches the maximum when r=r1. As for insulation design, the superconducting cable is designed so that the maximum electric field in the insulating layer 51a is lower than the withstand voltage performance thereof.
                    [                  Numerical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          1                ]                                                            E        =                  V                      r            ⁢                                                  ⁢                          ln              ⁡                              (                                                      r                    2                                    /                                      r                    1                                                  )                                                                        (        1        )            (V: voltage, r: radius, r1: inner radius of insulating layer, r2: outer radius of insulating layer)
On the other hand, Patent Document 2 discloses a configuration of an insulating layer which is called grading.
Unlike the insulating layer described in Patent Document 1 which is formed of the uniform insulating material, an insulating layer of grading has two layers. Among the two insulating layers, dielectric constant ∈1 of the inner insulating layer is high and dielectric constant ∈2 of the outer insulating layer is low.
By making the dielectric constant ∈1 of the inner insulating layer high and the dielectric constant ∈2 of the outer insulating layer low, it is possible to realize high withstand voltage performance considering AC characteristics.
The numerical expression 2 represents boundary voltage Va between the inner insulating layer and the outer insulating layer.
The voltage ΔV applied to the inner insulating layer is (V−Va). The voltage ΔV is small because the dielectric constant ∈1 of the inner insulating layer is made to be high and the dielectric constant ∈2 of the outer insulating layer is made to be low (∈1>∈2). As a result, the grading has an effect of alleviating (lowering) the inner electric field. Therefore, the maximum electric field directly on the conductor can be lowered to realize high withstand voltage performance.
                    [                  Numerical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          2                ]                                                                                      ⁢                              V            a                    =                                                    V                                  ɛ                  2                                            ⁢                              ln                ⁡                                  (                                                            r                      2                                                              r                      a                                                        )                                                                                                      1                                      ɛ                    1                                                  ⁢                                  ln                  ⁡                                      (                                                                  r                        a                                                                    r                        1                                                              )                                                              +                                                1                                      ɛ                    2                                                  ⁢                                  ln                  ⁡                                      (                                                                  r                        2                                                                    r                        a                                                              )                                                                                                          (        2        )                            ∈1: dielectric constant of inner insulating layer        ∈2: dielectric constant of outer insulating layer        r1: inner radius of inner insulating layer        r2: outer radius of outer insulating layer        ra: radius of border part between inner insulating layer and outer insulating layer        